High frame rate imaging theorem uses a pulsed plane wave in transmission and limited-diffraction array beam weightings are applied to received echo signals to produce a spatial Fourier transform of object function for 3D image reconstruction.
Because one transmission is used to construct an image, high image frame rate is achieved. In addition, because Fourier transform is implemented with a fast Fourier transform (FFT) which is a computationally efficient, simpler imaging system could be constructed to implement the method.
High frame rate imaging is important for imaging of fast moving objects such as the heart, especially, in three-dimensional (3D) imaging where many two-dimensional (2D) image frames are needed to form a 3D volume that may reduce image frame rate dramatically with conventional imaging methods.
Steered plane waves in transmissions have been used to increase image field of view, and reduce speckle noises. Also, limited-diffraction array beams in transmission have been used to increase field of view and spatial Fourier domain coverage to increase image resolution. Images constructed with different steering angles are combined with a coherent (enhancing resolution) or incoherent superposition (reducing speckles).
To increase field of view, a method using a spherical wave transmission followed by Fourier transformation for image reconstruction has also been proposed. Although this method may maintain a high frame rate at a large field of view due to the divergence nature of spherical waves, it may lower signal-to-noise ratio (SNR) and reduce computation efficiency as compared with the high frame rate imaging method.
The theory of high frame rate imaging and its extension have connections to many previous studies where Fourier transform was also used for ultrasonic imaging in the past two decades. However, the previous studies are not aimed at increasing the frame rate of conventional B-mode images. For example, a Fourier-domain reconstruction method for synthetic focusing was developed that solved the inverse scattering problem. A point source was used to transmit a broadband spherical wave over any given geometrical surfaces. 2D or 3D images are reconstructed using Fourier transformation. Apparently, the imaging process is slow. Another method used a Fourier based method using a fixed focus transmission and reception (cofocal) approach. This method requires multiple transmissions to cover the Fourier space of object function and thus is slow.
Another method used a plane wave steered at different angles to form a line of data in the Fourier space. Unfortunately, this method also requires a large number of transmissions to construct a frame of image.
Still another method used a narrow-band imaging method based on ultrasound holography and synthetic aperture concept, while another method also applied synthetic aperture focusing with Fourier transform to get C-mode images (the image plane is in parallel with the surface of a planar transducer).
One drawback of synthetic aperture methods is that they suffer from low, transmission efficiency since only part of the aperture of a transducer is used. Because a large amount of transmissions is required to construct an image, the image frame rate is low in addition to poor quality due to a low SNR. Nonetheless, Fourier-based synthetic aperture imaging is used with catheter-based ultrasound probes where a complex method is difficult to implement due to the confined space of these probes. Still another method used a Fourier-based 3D imaging method with mechanically scanning of a highly focused single-element transducer. Although the method mal get a high-resolution image beyond the focal distance and may, have applications in ophthalmology, and dermatology, it is not suitable for high frame rate imaging because mechanical scanning is usually very slow. Yet another method used suggested an imaging method that could be used to improve image resolution and contrast by transmitting multiple plane waves to coherently synthesize the so-called “sinc waves”. However, their method uses a time-domain approach and the complexity of an imaging system would be formidably high if it were applied to a 3D imaging at a rate of a few thousands volumes/s.
Therefore, there is a compelling and crucial need in the art for high-quality fast 3D ultrasound imaging that is made and operated at low, cost.